Probing microscopic variations of superconductivity on the surface of Ba(Fe1-xCox)2As2 single crystals

The spatially resolved electrical transport properties have been studied on the surface of optimally doped superconducting Ba(Fe1-xCox)(2)As-2 single crystal by using a four-probe scanning tunneling microscopy. While some nonuniform contrast appears near the edge of the cleaved crystal, the scanning electron microscopy (SEM) reveals mostly uniform contrast. For the regions that showed uniform SEM contrast, a sharp superconducting transition at T-C=22.1 K has been observed with a transition width Delta T-C=0.2 K. In the nonuniform contrast region, T-C is found to vary between 19.6 and 22.2 K with Delta T-C from 0.3 to 3.2 K. The wavelength dispersive x-ray spectroscopy reveals that Co concentration remains 7.72% in the uniform region, but changes between 7.38% and 7.62% in the nonuniform region. Thus the variations of superconductivity are associated with local compositional change.